The present invention relates generally to fabrication methods for solar cells, and more particularly to a novel method for bonding protective covers to an active surface of a solar cell.
The basic structure and operation of solar cells is well known and the use of silicon solar cells for electrical power generation for remote marine, terrestrial and satellite applications is well documented. The development of vertical junction solar cells met a need for radiation resistant cells with improved electrical conversion efficiency for operation in an ionizing radiation environment to which a cell may be subjected in extraterrestrial applications. The state of the art relating to vertical junction solar cells may be exemplified by solar cell configurations disclosed by or referenced in U.S. Pat. No. 3,690,953 titled "Vertical Junction Hardened Solar Cell", U.S. Pat. No. 3,985,579 titled "Rib and Channel Vertical Multijunction Solar Cell", U.S. Pat. No. 4,409,423 titled "Hole Matrix Vertical Junction Solar Cell", and U.S. Pat. No. 4,420,650 titled "Wedged Channel Vertical Junction Silicon Solar Cell".
Vertical junction solar cell structures known in the prior art may comprise a variety of configurations defining the vertical junctions of the solar cells. One such configuration includes a plurality of narrow grooves or channels etched into the surface of a semiconductor substrate, the channels providing the areas upon which the photovoltaic junction of the cell is diffused, the narrow ribs of the structure which define channels providing the vertical junctions for the cell. Another configuration is a silicon chip having in one surface a plurality of short, wedge shaped channels closely spaced in an array and separated by thin vertical ribs, the photovoltaic junction being diffused over the rib surfaces and chip surfaces between the ribs. Another cell configuration includes a silicon chip having in one surface a plurality of holes closely spaced in an array, the vertical junction areas being provided on the walls defining the holes.
The structures of the solar cells such as those just described may be exceedingly fragile, which often necessitates the bonding of a protective plate over the active surfaces of the solar cell in order to protect the photovoltaic junction areas of the cell from damage resulting from excessive heat or pressure, from physical impact, or from excessive or invasive radiation.
The invention is directed to a method of bonding a protective cover, such as of glass, to a solar cell to minimize vulnerability of the cell to damage during fabrication as well as during service. In the practice of the invention, a thin layer of metal is evaporated onto the solar cell chip in preselected configuration corresponding to that of the electrical gridwork of the completed solar cell; the thicker metallic current carrying electrical gridwork of the solar cell is deposited separately on a substrate and impressed into the protective cover; the cover containing the gridwork is then pressed onto and bonded to the solar cell chip with the gridwork in the cover in registration with the thin metal layer on the solar cell chip. The bonding of cover to chip in the process of the invention may be performed at a lower temperature than that required in conventional electrostatic bonding, which permits successful bonding and cell assembly without impairing the electrical performance of the solar cell.
It is therefore a principal object of the invention to provide an improved solar cell fabrication method.
It is a further object of the invention to provide an improved method for bonding a protective cover to a solar cell.
It is another object of the invention to provide a low temperature, low pressure method for bonding protective covers to solar cells.
It is yet another object of the invention to provide an improved solar cell structure.
These and other objects of the invention will become apparent as the description of representative embodiments proceeds.